1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading an image used for facsimile apparatus, digital copying machines, etc.
2. Description of the Related Art
An image reading apparatus, which converts an image into an electric signal for output, is used in order to transmit to a remote place and store an image on an original document.
Such an image reading apparatus is generally so constructed that an image on an original document to be read is transferred from a specified loading position to a reading position, the image on the original document is exposed by a light source such as a fluorescent lamp at the reading position, and a reflected light from the original document is converted into an electric signal by an image sensor such as a CCD line sensor.
Also in the image reading apparatus, an electrical process of shading correction is performed to remove an ununiform level in an image signal which is caused by an ununiform quantity of light from a light source, ununiform sensitivity of the image sensor and characteristics of an optical system such as a lens and a mirror.
To perform shading correction for a read signal in an image reading apparatus, a light volume distribution data for the above light source has been conventionally sampled. For this light volume distribution data, a reference plane, such as a white background, provided near a reading position is read in accordance with a detection that an original document has reached a position before the reading position, prior to reading an image on the original document, and data read from the reference plane is used. When a plurality of original documents are automatically fed continuously, the current position of the original document is checked to read the above reference plane immediately before each original document is fed to the reading position.
When, however, an oblique original has been conveyed, or an original at an angle, detection of the position to sample data for the above-mentioned shading correction has been delayed and as a result, there was a defect in a conventional apparatus that one part of the end of the original is erroneously read as the reference plane.
To eliminate such a defect, it may be considered to greatly delay the timing for sampling the light volume distribution data from the reference plane and the timing of arriving at the reading position for original documents by prolonging the conveyance interval of a plurality of original documents. In this case, however, a new defect occurs that it will take more time to read the entire plurality of original documents.
It has been proposed in commonly assigned U.S. Pat. Nos. 4,717,963 and 4,870,501 a configuration whereby shading correction is performed for image data in accordance with the presence of an original at a reading position.
In these documents, however, the presence of an original at a reading position is detected by using sensors such as a microswitch.